This invention relates to preparing copper surfaces for bonding to substrates used in the manufacture of printed circuit boards (PCB's). More particularly, the invention relates to the manufacture of multilayer PCB's.
Multilayer PCB's are constructed by interleafing imaged conductive layers of copper with dielectric layers to make a multilayer sandwich. The dielectric layers are organic resin layers that bond the copper layers together. The layers of copper and dielectric are bonded together by the application of heat and pressure. The surface of the copper is smooth and does not bond easily to the dielectric layer.
Improved bonding can be achieved by etching or otherwise roughening the surface of the copper to provide microscopic crevices and ridges in the surface of the copper. The copper surface may be roughened mechanically. However, delicate circuit patterns are susceptible to damage if mechanically roughened. Thus, there is a need for a copper surface roughening process that does not require mechanical roughening of the copper surface.
Oxide processes are also known in which an oxide having a rough surface is formed on the copper surface. The oxide may be formed by chemical treatment of the copper. Oxide processes have many shortcomings. A typical oxide process is run at such high temperatures that the substrate is often distorted, leading to quality control problems and additional production costs. The oxidation process is also associated with uniformity problems in which portions of the copper surface are not oxidized or coated by the oxidizing solution. Uniformity problems lead to partial delamination in the multilayer PCB's. To avoid this problem the PCB is run through multiple passes to obtain a more uniform oxide coating. Performing multiple passes adds considerably to production cost. Thus, there is a need for a copper roughening process that does not require multiple passes or high temperature.
Multilayer PCBs tend to have a sharp transition between each layer of etched copper surface and each layer of organic resin. Thus, there is a need for a coating promoter to facilitate the adhesion of the organic resin to the roughened copper surface.
U.S. Pat. No. 4,512,818 describes a treatment solution for the formation of a black oxide on copper surfaces of multi-layered printed circuits. The treatment solution comprises an oxidant and a hydroxide and is characterized by the addition of a water soluble or dispersible polymer to regulate the properties of the black oxide solution.
It is reported that major drawbacks of the black oxide procedure include marginal bond strengths and high temperature processing. Resulting surface coatings are prone to mechanical damage and partial delamination around through-holes; this problem is called “pink ring.” Pink ring is caused by the removal of the bonding oxide layer by through-hole cleaning and electroplating chemicals. Thus, there is a need for a formulation that is less prone to cause pink ring problems.
U.S. Pat. No. 5,861,076 describes a bond enhancement process for promoting strong, stable adhesive bonds between surfaces of copper foil and adjacent resin impregnated substrates or superimposed metallic sublayers. According to the process of the invention, a black oxide-coated copper surface is treated with an aqueous reducing solution containing sodium metabisulfite and sodium sulfide to convert the black oxide coating to a roughened metallic copper coating. The roughened metallic copper-coated surface is then passivated and laminated to a resin impregnated substrate.
In the chemical oxide modification process a strong reducing agent, typically dimethylamine borane, is applied to the oxide coating to obtain an even oxide coating. This type of adhesion promotion process produces an oxide coating that is fragile and prone to scratching during handling. Inspection of the circuitry prior to lamination is difficult because of the fragility of the oxide coating. Therefore, there is a need for an adhesion promotion process that permits a less problematic inspection after the adhesion promotion process and prior to the lamination step.
A method for roughening copper or copper alloy surfaces is described in U.S. Pat. No. 5,532,094. The copper surface is treated with an aqueous solution comprising an azole compound, a soluble copper compound, an organic or inorganic acid and a halogen ion.
U.S. Pat. No. 5,073,456 is directed to the manufacture of multilayer printed circuit boards having a number of through-holes that are formed by employing intermediate layers when bonding copper circuitry to an insulating layer.
U.S. Pat. Nos. 3,756,957, and 5,800,859 describe a range of hydrogen peroxide stabilizers; U.S. Pat. Nos. 3,756,957 and 5,800,859 are hereby incorporated by reference in their entirety. Alpha Metals sells an adhesion promoting solution under the registered trademark “Alpha Prep.”
U.S. Pat. No. 5,800,859 describes a process for providing copper coating printed circuit boards. The process includes a treating step in which a metal surface is contacted with an adhesion promotion material. The adhesion promotion material includes 0.1 to 20% by weight hydrogen peroxide, an inorganic acid, an organic corrosion inhibitor and a surfactant. The surfactant is preferably a cationic surfactant, usually an amine surfactant and most preferably a quaternary ammonium surfactant.